Damped fiber optic accelerometers, sensors, and sensor assemblies, and methods of assembling the same

ABSTRACT

A fiber optic sensor is provided. The fiber optic sensor includes: a fixed portion configured to be secured to a body of interest; a moveable portion; a spring member positioned at least partially between the fixed portion and the moveable portion; an optical fiber wound in contact with the fixed portion and the moveable portion such that the optical fiber spans at least a portion of the spring; and an elastomeric material provided in contact with at least one of the fixed portion, the moveable portion, the spring member, the body of interest, and the optical fiber.

RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 62/014,319, filed on Jun. 19, 2014, thecontents of which are incorporated in this application by reference.

FIELD

The present invention relates to the fiber optic sensing, and moreparticularly, to fiber optic sensors with improved dampingcharacteristics.

BACKGROUND

Fiber optic accelerometers are typically resonant devices in that theyhave a high quality factor (‘Q’) frequency response which can limit thedynamic range when mechanical inputs are broadband and span the naturalfrequency of the accelerometer. While a desirable a fiber optic sensorhas a constant frequency response, in practice the frequency responsemay have a highly variable magnitude, resulting in a limited dynamicrange.

Using exemplary aspects of the present invention, it would be desirableto reduce the variability of the frequency response of the fiber opticsensor, thereby improving both the dynamic range and the variability ofthe low frequency scale factor, and extending the useable bandwidth ofthe accelerometer to higher frequencies.

SUMMARY

According to an exemplary embodiment of the present invention, a fiberoptic sensor is provided. The fiber optic sensor includes: a fixedportion configured to be secured to a body of interest; a moveableportion; a spring member positioned at least partially between the fixedportion and the moveable portion; an optical fiber wound in contact withthe fixed portion and the moveable portion such that the optical fiberspans at least a portion of the spring; and an elastomeric materialprovided in contact with at least one of the fixed portion, the moveableportion, the spring member, the body of interest, and the optical fiber.

According to another exemplary embodiment of the present invention, afiber optic sensor assembly is provided. The fiber optic sensor assemblyincludes: a fixed portion configured to be secured to a body ofinterest; a moveable portion; a spring member positioned at leastpartially between the fixed portion and the moveable portion; an opticalfiber wound in contact with the fixed portion and the moveable portionsuch that the optical fiber spans at least a portion of the spring; anelastomeric material provided in contact with at least one of the fixedportion, the moveable portion, the spring member, the body of interest,and the optical fiber; and a housing configured to receive each of thefixed portion, the moveable portion, the spring member, the opticalfiber, and the elastomeric material.

According to yet another exemplary embodiment of the present invention,a method of assembling a fiber optic sensor is provided. The methodincludes the steps of : (a) providing a fixed portion configured to besecured to a body of interest, a moveable portion, a spring memberpositioned at least partially between the fixed portion and the moveableportion, and an optical fiber wound in contact with the fixed portionand the moveable portion such that the optical fiber spans at least aportion of the spring; and (b) providing an elastomeric material to bein contact with at least one of the fixed portion, the moveable portion,the spring member, the body of interest, and the optical fiber. Step (b)may include, for example: (1) applying the elastomeric material as afluid to be in contact with at least one of the at least one of themoveable portion and the spring member; (2) applying the elastomericmaterial as a fluid to surround each of the moveable portion and thespring; (3) providing the elastomeric material as at least one solidelastomeric element in contact with at least one of the at least one ofthe moveable portion and the spring; or (4) providing the elastomericmaterial as a plurality of solid elastomeric elements in contact with atleast one of the at least one of the moveable portion and the spring,amongst other methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1A is a block diagram of a fiber optic sensing system in accordancewith an exemplary embodiment of the present invention;

FIG. 1B is a block diagram of detailed portion of the fiber opticsensing system of FIG. 1A;

FIG. 2 is a block diagram of the fiber optic sensing system of FIG. 1Ain a downhole application in accordance with an exemplary embodiment ofthe present invention;

FIG. 3 is a graphical illustration of frequency responses ofconventional and inventive fiber optic sensors in accordance with anexemplary embodiment of the present invention; and

FIGS. 4A-4H are block diagram illustrations of various fiber opticaccelerometers, and components of fiber optic accelerometers, inaccordance with various exemplary embodiments of the present invention.

DETAILED DESCRIPTION

In accordance with certain exemplary embodiments of the presentinvention, an elastomeric (e.g., rubber like) material is provided incontact with (and in some cases surrounding) certain parts of a fiberoptic sensor (e.g., a fiber optic accelerometer). For example, theelastomeric material may be applied as a fluid material (e.g., that maybe cured, for example, with or without the application or heat, light orother energy) or as one or more solid elastomeric components.

In certain exemplary embodiments of the present invention, a moveableportion of an accelerometer (and/or a mass engaged with the moveableportion) and/or other portions of the accelerometer are “potted” orotherwise engaged with (e.g., in contact with) an elastomeric materialin a way that reduces the magnitude of the peak response of theaccelerometer, thereby significantly reducing the Q, and thereforeimproving both the dynamic range and the consistency of the lowfrequency scale factor (sensitivity) below resonance. For example, theelastomeric material may surround or be in contact with certain elementsof the accelerometer to provide the desired damping function. Specificexamples for use of the elastomeric material include: (1) theelastomeric material surrounding the entire accelerometer; (2) theelastomeric material surrounding one or more elements of theaccelerometer, such as the moveable portion; (3) the elastomericmaterial in contact with one or more elements of the accelerometer, suchas the moveable portion; (4) the elastomeric material being positionedbetween elements of the accelerometer, such as between the moveableportion and a mass engaged with the moveable portion, or between themoveable portion and a fixed portion, amongst other combinations; or (5)the elastomeric material partially or completely filling gaps betweenelements of the accelerometer such as between the moveable portion and amass engaged with the moveable portion, or between the moveable portionand a fixed portion, amongst other combinations.

As used herein, the term elastomeric materials is intended to be broadlydefined as a material selected from the group consisting of rubbers,neoprenes, urethanes, epoxies, silicones and viscoelastic materials. Incertain exemplary embodiments of the present invention, the elastomericmaterial is a viscoelastic (lossy) material.

FIG. 1A illustrates a fiber optic sensing system 100. Fiber opticsensing system 100 includes interrogation electronics 102, a lead cable104, and a fiber optic sensor array 106. Fiber optic sensor array 106includes a plurality of fiber optic sensing tools 106 a, 106 b, . . . ,106 n. Interconnect fiber optic cable 108 is provided between fiberoptic sensing tools 106 a, 106 b, . . . , 106 n. Interrogationelectronics 102 includes an optical source (e.g., a light source such asan LED, etc.) for providing light to fiber optic sensor array 106. Thelight is received back at interrogation electronics using an opticalreceiver. For example, each of the tools 106 a, 106 b, . . . , 106 nincludes one or more fiber optic sensors (e.g., accelerometers), wherethe sensors convert mechanical or physical motion (such as acceleration)to a change in the strain (e.g., longitudinal strain) in an opticalfiber. At each of the sensors, the change in strain may then beconverted to a change in the phase of light that passes through theoptical fiber. Interrogation electronics 102 is able to analyze thechange in the phase of light to determine information related to theapplication (e.g., environment) of the fiber optic sensor array such as,for example: vertical seismic profiling (VSP), three dimensionalsub-surface mapping, microseismic monitoring, machine vibrationmonitoring, civil structure (e.g., dams, bridges, levees, buildings,etc.) monitoring, tunnel detection, perimeter/border security,earthquake monitoring, borehole leak detection, roadbed erosion, railbederosion, pipeline monitoring, amongst other applications.

FIG. 1B illustrates details of an exemplary fiber optic sensing tool 106a (e.g., a fiber optic sensor assembly). Tool 106 a includes a housing106 a 1. Lead cable 104 (or another fiber optic cable) is opticallycoupled as an input to tool 106 a, and interconnect fiber optic cable108 is optically coupled as an output from tool 106 a (leading to tool106 b). Within housing 106 a 1 are three (3) fiber optic sensors 106 a2, 106 a 3, and 106 a 4. For example, each of sensors 106 a 2, 106 a 3,and 106 a 4 may be configured to sense mechanical or physical motionalong a specific axis (e.g., as illustrated, sensor 106 a 2 isconfigured to sense along the x-axis, sensor 106 a 3 is configured tosense along the y-axis, and sensor 106 a 4 is configured to sense alongthe z-axis). As will be appreciated by those skilled in the art, the useof three (3) fiber optic sensors in tool 106 a is exemplary in nature,and may be varied as desired in the specific application.

FIG. 2 illustrates fiber optic sensing system 100 in a downholeapplication, for example, in an application configured to sense seismicinformation related to the oil and gas industry. In the example shown inFIG. 2, interrogation electronics 102 is above ground, while fiber opticsensing array 106 is disposed in a borehole 202 in the earth 200. Leadcable 104 optically connects interrogation electronics 102 to fiberoptic sensing array 106. In the illustration of FIG. 2, each of tools106 a, 106 b, . . . , 106 n includes a respective clamping systemincluding clamp arm 106 a 5, 106 b 5, . . . , 106 n 5. In FIG. 2, clampsarms 106 a 5, 106 b 5, . . . , 106 n 5 are in an extended positionproviding the respective tools in a substantially fixed position inborehole 202, pressed against a sidewall of borehole 202. Clamp arms 106a 5, 106 b 5, . . . , 106 n 5 are configured to be operated between aretracted position (e.g., during lowering of array 106 into borehole202, and removal of array 106 from borehole 202) and a extended position(as shown in FIG. 2) during sensing operations.

FIG. 3 illustrates a frequency response of various fiber optic sensors(e.g., fiber optic accelerometers). The upper curve (shown as a solidline) illustrates an example undamped mass-spring fiber optic sensor(accelerometer) with a relatively constant sensitivity (in rad/g) atlower frequencies. At the sensor's natural frequency, the mechanicalresonance naturally causes the sensitivity to peak sharply which mayresult in a limited dynamic range.

As shown in the lower 2 curves in FIG. 3, by increasing the damping ofthe fiber optic sensor, the magnitude of the peak sensitivity decreasesrelative to the low frequency sensitivity, thereby providing a moreconstant frequency response and an improved dynamic range.

FIGS. 4A-4H illustrate various fiber optic accelerometers, andcomponents of fiber optic accelerometers, in accordance with variousexemplary embodiments of the present invention. Exemplary fiber opticaccelerometers are disclosed in U.S. Patent Application Publication No.2012/0257208, titled “FIBER OPTIC TRANSDUCERS, FIBER OPTICACCELEROMETERS AND FIBER OPTIC SENSING SYSTEMS”, which is herebyincorporated by reference in its entirety.

Elements having the same reference numerals in the drawings shall beconsidered to be same element described throughout the presentapplication unless specified otherwise.

Referring specifically to FIG. 4A, a fiber optic sensor 106 a 2 (e.g.,an accelerometer) is secured to a body of interest 400. Fiber opticsensor 106 a 2 includes a fixed portion 402 secured to body of interest400, a moveable portion 404 that moves along at least one axis (within arange of motion) with respect to fixed portion 402, and a spring member406 positioned between fixed portion 402 and moveable portion 404. Anoptical fiber 108 a (e.g., an optical fiber from fiber optic cable 108shown in FIG. 1B) is wound around and between fixed portion 402 andmoveable portion 404 such that optical fiber 108 a spans spring member406. The ends of optical fiber 108 a are optically connected (e.g.,through fiber optic cable 108, lead cable 104, other fiber opticcomponents comprising an interferometer, etc.) to interrogationelectronics 102 shown in FIG. 1 such that optical signals may bereceived at fiber optic sensor 106 a 2 from an optical source ininterrogation electronics 102.

Elastomeric material 408 is provided in contact with various elements offiber optic sensor 106 a 2 a. In fact, in FIG. 4A, elastomeric material408 surrounds all (or at least a portion of) fiber optic sensor 106 a 2(and the various elements included in fiber optic sensor 106 a 2). Thishas a damping effect on fiber optic sensor 106 a 2, thereby improvingit's dynamic range, amongst other benefits.

FIG. 4B illustrates the same elements as in FIG. 4A, except that fiberoptic sensor 106 a 2 a is provided within a cavity 410 of body ofinterest 400. As shown, with fiber optic sensor 106 a 2 a provided incavity 410, substantially all of the remaining open area within cavity410 is filled with elastomeric material 408—such that elastomericmaterial 408 surrounds and is attached to all (or at least a portion of)fiber optic sensor 106 a 2 a.

FIG. 4C illustrates the same elements as in FIG. 4A, except that infiber optic sensor 106 a 2 b elastomeric material 408 is provided in areduced volume, to surround spring member 406—and to be in contact withfixed portion 402, moveable portion 404, and optical fiber 108 a.

FIG. 4D illustrates the same elements as in FIG. 4A, except that: anadditional mass 404 a is engaged with (e.g., connected to) moveableportion 404; and elastomeric material 408 is provided in a reduced area,to be in contact with a portion of spring member 406, moveable portion404, optical fiber 108 a, and mass 404 a. Mass 404 a moves with moveableportion 404 along the motion axis (or axes) of moveable portion 404, andmay be provided to increase the sensitivity of fiber optic sensor 106 a2 c.

FIG. 4E illustrates similar elements as in FIG. 4 k However, in FIG. 4Efiber optic sensor 106 a 2 d includes a fixed portion 402 a, springmember 406 a, and moveable portion 404 b formed from a unitary piece ofmaterial. Further, an additional mass 404 c is provided engaged with(e.g., connected to) moveable portion 404 b, and includes side wallportions 404 c 1 at least partially surrounding at least one of springmember 406 a, the fixed portion 402 a, and other elements of fiber opticsensor 106 a 2 d as shown in FIG. 4E, during at least one positionwithin the range of motion of moveable portion 404 b. Further still,body of interest 400 b defines cavity 404 b 1. Fiber optic sensor 106 a2 d is positioned at least partially within cavity 400 b 1. Elastomericmaterial 408 is provided in an open area within cavity 400 b 1 (andperhaps filling the open area of cavity 400 b 1) such that elastomericmaterial 408 surrounds and is attached to all (or at least a portion of)fiber optic sensor 106 a 2 d.

In each of FIGS. 4A-4E, elastomeric material 408 is provided as a fluidmaterial applied to the desired volume of the respective fiber opticsensor. This fluid material cures, providing the desired elastomeric(damping) effect. However, in accordance with certain exemplaryembodiments of the present invention, solid elastomeric materials may beapplied (e.g., positioned) in connection with the respective fiber opticsensor. For example, in FIG. 4F, a solid core elastomeric member 418(e.g., a rubber rod) is provided within and may be attached to a springmember 406 b. In another example in FIG. 4G, a spring member 406 c isprovided within and may be attached to a hollow elastomeric member 428(e.g., a rubber tube). In either case, the combined spring member andelastomeric member may be included in a fiber optic sensor (e.g., suchas the sensors illustrated and described in connection with FIGS.4A-4D), taking the place of spring members 406 and elastomeric material408.

FIG. 4H illustrates an exemplary fiber optic sensor 106 a 2 e secured toa body of interest 400. Fiber optic sensor 106 a 2 e includes a combinedfixed portion 402 a, spring member 406 a, and moveable portion 404 bformed from a unitary piece of material as in FIG. 4E. Further, anadditional mass 404 d is provided engaged with (e.g., connected to)moveable portion 404 b. In FIG. 4H, solid elastomeric material elements408 a, 408 b, and 408 c (e.g., rubber members, such as shims) areprovided in between ones of the fixed portion 402 a, spring member 406a, and moveable portion 404 b to provide the desired damping function.

According to certain exemplary embodiments of the present invention,methods of assembling fiber optic accelerometers/sensors and sensorassemblies (including a housing, as in FIG. 1B) are provided. As will beappreciated by those skilled in the art, certain steps included in themethods described below may be omitted; certain additional steps may beadded; and the order of the steps may be altered from the orderdescribed.

In an example where the fixed portion, the spring member, and themoveable portion of an accelerometer are formed from a unitary piece ofmaterial, and the elastomeric material is applied as a fluid (such as inFIG. 4E), an example assembly method includes: (1) providing the unitarypiece of material including a fixed portion, a moveable mandrel and aspring member therebetween; (2) winding a single length of optical fiberbetween (and around) the fixed portion and the moveable portion; (3)optionally attaching a proof mass to the moveable portion; (4) applying(e.g., using a syringe) a fluid elastomeric material (e.g., a liquidresin) in selected volumes/areas between elements of the accelerometersuch as between ones of the moveable portion, the spring member, thewound optical fiber, and the proof mass; (5) curing and/or vulcanizingthe elastomeric material to become a flexible, solid elastomer (e.g.,depending upon the fluid elastomeric material used, some form of energymay be used in the curing process such as heat, light, a combination ofheat and force, etc.); and (6) attaching the accelerometer to the bodyof interest.

In an example where the fixed portion, the spring member, and themoveable portion of an accelerometer are formed from a unitary piece ofmaterial, and the elastomeric material is applied as a solid (such as inFIG. 4H), an example assembly method includes: (1) providing the unitarypiece of material including a fixed portion, a moveable mandrel and aspring member therebetween; (2) inserting one or more flexible, solidelastomeric elements in gaps between structural elements of the unitarypiece of material; (3) winding a single length of optical fiber between(and around) the fixed portion and the moveable portion; (3) optionallyattaching a proof mass to the moveable portion; (4) attaching theaccelerometer to the body of interest.

In an example where the fixed portion, the spring member, and themoveable portion of an accelerometer are separate elements, and theelastomeric material is applied as a fluid (such as in FIGS. 4A-4D), anexample assembly method includes: (1) providing a fixed portion, amoveable mandrel and a spring member therebetween; (2) winding a singlelength of optical fiber between (and around) the fixed portion and themoveable portion; (3) optionally attaching a proof mass to the moveableportion; (4) applying (e.g., using a syringe) a fluid elastomericmaterial (e.g., a liquid resin) in selected volumes/areas betweenelements of the accelerometer such as between ones of the moveableportion, the spring member, the wound optical fiber, and the proof mass;(5) curing and/or vulcanizing the elastomeric material to become aflexible, solid elastomer (e.g., depending upon the fluid elastomericmaterial used, some form of energy may be used in the curing processsuch as heat, light, a combination of heat and force, etc.); and (6)attaching the accelerometer to the body of interest.

In an example where the fixed portion, the spring member, and themoveable portion of an accelerometer are separate elements, and theelastomeric material is applied as a solid (such as accelerometersincluding the elements shown in FIGS. 4F-4G), an example assembly methodincludes: (1) providing a fixed portion, a moveable mandrel and a springmember therebetween; (2) inserting one or more solid elastomericelements in spaces/gaps/areas between elements of the acceleromter suchas between the spring member and another element of the accelerometer;(3) winding a single length of optical fiber between (and around) thefixed portion and the moveable portion; (4) optionally attaching a proofmass to the moveable portion; (5) attaching the accelerometer to thebody of interest.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. A fiber optic sensor comprising: a fixed portionconfigured to be secured to a body of interest; a moveable portion; aspring member positioned at least partially between the fixed portionand the moveable portion; an optical fiber wound in contact with thefixed portion and the moveable portion such that the optical fiber spansat least a portion of the spring; and an elastomeric material providedin contact with at least one of the fixed portion, the moveable portion,the spring member, the body of interest, and the optical fiber.
 2. Thefiber optic sensor of claim 1 further comprising a mass coupled to themoveable portion, the mass configured to move with the moveable portionalong at least one motion axis of the moveable portion.
 3. The fiberoptic sensor of claim 1 wherein the fixed portion, the spring member,and the moveable portion are formed from a unitary piece of material. 4.The fiber optic sensor of claim 1 wherein the moveable portion includesa mass that at least partially surrounds at least one of the springmember and the fixed portion during at least one position within therange of motion of the moveable portion.
 5. The fiber optic sensor ofclaim 1 wherein the elastomeric material is provided in contact witheach of the moveable portion, the spring member, and the optical fiber.6. The fiber optic sensor of claim 1 wherein the elastomeric materialsurrounds at least one of the moveable portion and the spring.
 7. Thefiber optic sensor of claim 1 wherein the elastomeric material surroundseach of the moveable portion and the spring.
 8. The fiber optic sensorof claim 1 wherein the elastomeric material is provided as at least onesolid element during assembly of the fiber optic sensor.
 9. The fiberoptic sensor of claim 1 wherein the elastomeric material is provided asa fluid during assembly of the fiber optic sensor.
 10. The fiber opticsensor of claim 1 wherein the elastomeric material includes a materialselected from the group consisting of rubbers, neoprenes, urethanes,epoxies, silicones and viscoelastic materials.
 11. The fiber opticsensor of claim 1 wherein the elastomeric material is a viscoelasticmaterial.
 12. The fiber optic sensor of claim 1 wherein the fiber opticsensor is a fiber optic accelerometer.
 13. A fiber optic sensor assemblycomprising: a fixed portion configured to be secured to a body ofinterest; a moveable portion; a spring member positioned at leastpartially between the fixed portion and the moveable portion an opticalfiber wound in contact with the fixed portion and the moveable portionsuch that the optical fiber spans at least a portion of the spring; anelastomeric material provided in contact with at least one of the fixedportion, the moveable portion, the spring member, the body of interest,and the optical fiber; and a housing configured to receive each of thefixed portion, the moveable portion, the spring member, the opticalfiber, and the elastomeric material.
 14. The fiber optic sensor assemblyof claim 13 further comprising a clamping system engaged with thehousing, the clamping system including a clamp arm configured to beoperated between an extended position and a retracted position duringengagement in a wellbore.
 15. A method of assembling a fiber opticsensor, the method comprising the steps of: (a) providing a fixedportion configured to be secured to a body of interest, a moveableportion, a spring member positioned at least partially between the fixedportion and the moveable portion, and an optical fiber wound in contactwith the fixed portion and the moveable portion such that the opticalfiber spans at least a portion of the spring; and (b) providing anelastomeric material to be in contact with at least one of the fixedportion, the moveable portion, the spring member, the body of interest,and the optical fiber.
 16. The method of claim 15 wherein step (b)includes applying the elastomeric material as a fluid to be in contactwith at least one of the at least one of the moveable portion and thespring.
 17. The method of claim 15 wherein step (b) includes applyingthe elastomeric material as a fluid to surround each of the moveableportion and the spring.
 18. The method of claim 15 wherein step (b)includes providing the elastomeric material as at least one solidelastomeric element in contact with at least one of the at least one ofthe moveable portion and the spring.
 19. The method of claim 15 whereinstep (b) includes providing the elastomeric material as a plurality ofsolid elastomeric elements in contact with at least one of the at leastone of the moveable portion and the spring.
 20. The method of claim 15wherein the elastomeric material provided in step (b) includes amaterial selected from the group consisting of rubbers, neoprenes,urethanes, epoxies, silicones and viscoelastic materials.
 21. The methodof claim 15 wherein the elastomeric material is a viscoelastic material.